This invention relates in general to protection devices for protecting circuits against excess current and voltage, and in particular to a protection device for a coaxial cable.
Coaxial cable is in widespread use for transmitting signals, particularly over cable television lines. A coaxial cable has a center conductor that is insulated. An outer conductor, which may be foil, woven, or multiple layers of both, surrounds the inner conductor insulation. Although it has been a longstanding practice to provide a protection device at the junction box between the telephone company lines, which are twisted-pairs, and equipment in a home or business, this has not been widely used with coaxial distribution lines. For telephone cable lines, the protection device includes an excess voltage protector that conducts to a ground when encountering excessive voltage. The excess voltage protector may be of a gas tube type or solid state. For excess current protection, a fuse will be provided.
Overcurrent protection devices have been used to some extent for coaxial cables. One prior practice has been to connect into the line a relatively long length, approximately 20 inches, of coaxial cable with a center conductor wire that has a gage two or three sizes smaller than the gage of the network center conductor wire. This technique is not completely reliable as the coaxial cable intended to be a fuse link does not always open in a predictable location. For example, the coaxial connector may inadvertently act as the fusible element, which is unsatisfactory. Another technique is to use a medium length coaxial cable, less than three inches, which has been designed with an extremely small gage center wire. This particular type is difficult to manufacture. Available overcurrent protection devices are usually contained in a physically separate package from overvoltage protection devices.
Another problem dealing with protection devices involves characteristic impedance mismatch. It is important to match the characteristic impedance of the protection device to the characteristic impedance of the transmission line, which in the case of coaxial cable for cable television applications is typically 75 ohms. Impedance mismatch may result in unacceptable insertion loss and return loss characteristics, which results in data loss. Overvoltage protection elements, such as air gaps, gas tubes, or solid state devices such as thyristors, have a capacitance that is often many magnitudes larger than the inherent capacitance of the transmission line in the network they are designed to protect. When these devices are inserted into the transmission line, the characteristic impedance of the network becomes mismatched in the area of the protector and signal losses occur.
In this invention, the fuse assembly comprises a trace formed on a thin, flat dielectric substrate, creating a printed circuit board. The trace has a length, width, and thickness that is designed to open if a selected current for a selected time duration is reached. The substrate preferably has a second side that is coated with a conductive layer having a greater cross-sectional area than the trace. The trace is connected in a series arrangement to the center conductor of a coaxial cable while the conductor layer on the opposite side is connected in a series arrangement to the outer conductor of the coaxial cable. The printed circuit board, connected in series with the inner and outer conductors of the coaxial cable becomes a microstrip transmission line with a characteristic impedance designed to match that of the coaxial cable. In the preferred embodiment, the printed circuit board is mounted and insulated within a housing.
Also, an excess voltage protector may be mounted in the housing, preferably in a chamber separate from the fuse. The excess voltage protector may be an air gap, gas tube, or a thyristor type protector.
The excess voltage protector has a capacitance that must be accounted for in matching the characteristic impedance of the protector to the coaxial cable transmission line. The overcurrent protector trace width, thickness, configuration, and circuit board material may be designed to provide a designed impedance match for the coaxial cable transmission line.
The key to providing a transmission line protector with low signal losses and reflections is in matching the characteristic impedance of every section of the protector with that of the transmission line it is intended to be used with. In the ideal configuration, the coaxial connectors, overcurrent protector, overvoltage protector, and transitional areas are designed with matching characteristic impedance. The characteristic impedance of a two-conductor transmission line is given by the following:       Zo    =                                        R            +                                          j                ·                2                            ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢              f              ⁢                              xe2x80x83                            ⁢              L                                            G            +                                          j                ·                2                            ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢              f              ⁢                              xe2x80x83                            ⁢              C                                          ⁢              xe2x80x83            ⁢              (        Ω        )              ,
where Zo is the characteristic impedance in ohms, f is the frequency in Hertz, j is the imaginary number, R is the resistance per unit length (both conductors) in ohms per meter, L is the inductance per unit length (both conductors) in Henries per meter. G is the conductance per unit length (between conductors) in Siemens per meter, and C is the capacitance per unit length (between conductors) in Farads per meter.
The characteristic impedance is unique at every cross-section in the transmission line; it is dependent on the physical dimensions, material properties, and the frequency of the signal. In the design of most protection the resistance and the conductance is extremely low and can be neglected. The equation for the characteristic impedance can then be approximated by:       Zo    =                                                      xe2x80x83                        ⁢            L                                              xe2x80x83                        ⁢            C                              ⁢              xe2x80x83            ⁢              (        Ω        )              ,
It can be seen from the above equation that the characteristic impedance is dependent on the ratio of the inductance to the capacitance of the transmission line. The connectors have a center-connector outer diameter, insulator shape, insulator material type, and outer-conductor inner diameter that all can be altered to achieve the proper characteristic impedance. The overcurrent protection printed circuit board has a trace width, thickness, configuration, and circuit board material and thickness that can be altered to design the proper characteristic impedance. The excess voltage protection device has a capacitance, that, when inserted into a coaxial transmission line, must be adjusted for in the design of the transmission line to ensure a characteristic impedance match. The physical addition of the excess voltage protection device into the transmission line also introduces undesirable inductance and capacitance effects that must be accommodated for in the design. When the excess voltage protection device is placed between the center and outer conductor of the coaxial transmission line, its capacitance is effectively in parallel with the inherent capacitance of the coaxial transmission line. This is adjusted for in the design by decreasing the inherent capacitance of the transmission line, increasing the inherent inductance of the transmission line, or both. In the preferred embodiment described here, this is accomplished by adjusting the inner conductor outer diameter, conductor material, dielectric material, outer conductor inner diameter, and excess voltage protection device placement.